In a typical wet-type multiple-disc clutch, friction plates and separator plates are alternately disposed between a clutch or brake drum and a hub. A clutch piston is pressed to engage the clutch or released to disengage the clutch.
In recent years, the demand for reducing the fuel consumption of an automobile is more than ever increasing. In an automatic transmission, accordingly, there has been ever-increasing demand for achieving a reduced drag torque between a friction plate and a separator plate in order to reduce a power loss when the clutch is in a disengagement mode.
In order to reduce a power loss, a typical wet-type multiple-disc clutch used with an automatic transmission (AT) is frequently configured to permit easy flow of a lubricating oil from an inner periphery side to an outer periphery side of the friction plate so as to reduce drag torque. As a technique for reducing the drag torque, there is known a technique described in, for example, PTL 1 or PTL 2. In the clutch described in PTL 1 and PTL 2, a friction plate is provided with an oil groove having an end surface thereof closed on the inner periphery side to keep the friction plate and a separator plate apart in the disengagement mode, and an oil passage for supplying lubricating oil to a friction surface to prevent seizure in an engagement mode, the oil passage penetrating in the direction of an inner diameter and an outer diameter.
In recent years, however, the clearance between a friction plate and a separator plate has been reduced, as compared with conventional clutches, in order to improve the responsiveness to a speed change so as to achieve improved power performance in addition to improved fuel economy. This has been leading to a tendency to increase the drag torque attributable to an intervening oil film at the time of idling.
Further, the oil supplied to the oil passage, which penetrates from the inner diameter side to the outer diameter side, is drawn toward a friction material due to rotation. Once the oil that has been drawn enters between a friction plate and a separator plate, it is difficult to drain the oil, especially in a small area with a smaller number of rotations in the clearance between the friction plate and the separator plate. This results in a large drag torque attributable to the viscosity between a friction material and a counterpart separator plate.